This invention relates to power converters and more specifically to interleaved direct-current to alternating-current power converters.
Power conversion problems are encountered when utilizing equipment designed for alternating current (AC) power in an environment where only direct current (DC) power is available or where a DC primary power link is purposely employed to provide a battery back up in the event AC power becomes unavailable. In these applications a DC-to-AC power converter is needed to interface the equipment to the primary power system. Virtually all modern AC equipment operates from either 115 or 230 volts, plus or minus 10 percent, at either 50, 60 or 400 Hz, plus or minus 5 percent. In general, DC primary power supplies are normally within the 11 to 32 volts DC range. This includes, for example, most modern aircraft and vehicular equipment.
In the prior art, the simplest approach to DC-to-AC power conversion was the use of a 60/400 Hz inverter followed by a low frequency transformer and filter. Regulation is achieved by preceding the inverter with an ultrasonic switching regulator or alternatively achieved in the inverter itself by varying the duty cycle of the inverter. In the case of the latter approach, the output filter must accommodate the worst case harmonic content of the signal's waveform which usually occurs at maximum input voltage. In the case of the former approach, the burden on the output filter is reduced because the DC level to the inverter is held constant. However, the switching regulator adds to the cost and degrades the efficiency of the overall power converter. In either case, the output filter must provide significant attenuation at the fundamental output frequency in order to provide a sinusoidal waveform.
The next level of sophistication of power converter design, in the prior art, is not only to vary the duty cycle of the inverter, but to chop the inverter's output signal into pulses with variable pulse widths by means of ultrasonic switching by using either feedback control against a sinusoidal reference or by "open loop programming" of the control to distribute the width of the pulses in a sinusoidal fashion, as described in U.S. Pat. No. 4,244,016 incorporated herein by reference. The burden of the output filter is thus, reduced. Sinusoidally distributing the ultrasonic switching allows the reduction in the size of the output filter and improves closed loop response. These improvements are a result of the higher break point in the frequency response of the output filter and the higher sample data rate of the inverter. However, this approach still requires a large low frequency power transformer. Especially in the application of 50 to 60 Hz AC power, the size of the transformer completely dominates the size and weight of the power converter.
A further level of sophistication of power converter design, in the prior art, was for the inverter to generate bi-directional pulses at the ultrasonic switching frequency, thereby permitting a much smaller power transformer, and to steer these pulses, by means of a secondary switching apparatus, either positively or negatively on a cycle-by-cycle basis, to synthesize the same sinusoidally distributed pulse-width-modulated signal for application to the high break point output filter as described in U.S. Pat. No. 4,339,791 incorporated herein by reference.
However, for any given ultrasonic switching frequency, there is a maximum output frequency for which faithful reproduction of the waveshape, sinusoidal or otherwise, can reasonably be achieved. Particularly in high power, high efficiency switching power amplifier applications, where modulation frequencies can be much higher than standard power line frequencies, approaching the ultrasonic range themselves, there exists a need for high power, even wider bandwidth power converters to track high frequency modulation envelopes independent of polarity.